NAME

icmp, IPPROTO_ICMP - Linux IPv4 ICMP kernel module.

DESCRIPTION

This kernel protocol module implements the Internet Control Message
Protocol defined in RFC 792. It is used to signal error conditions and
for diagnosis. The user doesn’t interact directly with this module;
instead it communicates with the other protocols in the kernel and
these pass the ICMP errors to the application layers. The kernel ICMP
module also answers ICMP requests.
A user protocol may receive ICMP packets for all local sockets by
opening a raw socket with the protocol IPPROTO_ICMP. See raw(7) for
more information. The types of ICMP packets passed to the socket can
be filtered using the ICMP_FILTER socket option. ICMP packets are
always processed by the kernel too, even when passed to a user socket.
Linux limits the rate of ICMP error packets to each destination.
ICMP_REDIRECT and ICMP_DEST_UNREACH are also limited by the destination
route of the incoming packets.

SYSCTLS

ICMP supports a sysctl interface to configure some global IP
parameters. The sysctls can be accessed by reading or writing the
/proc/sys/net/ipv4/* files or with the sysctl(2) interface. Most of
these sysctls are rate limitations for specific ICMP types. Linux 2.2
uses a token bucket filter to limit ICMPs. The value is the timeout in
jiffies until the token bucket filter is cleared after a burst. A jiffy
is a system dependent unit, usually 10ms on x86 and about 1ms on alpha
and IA64.
icmp_destunreach_rate
Maximum rate to send ICMP Destination Unreachable packets. This
limits the rate at which packets are sent to any individual
route or destination. The limit does not affect sending of
ICMP_FRAG_NEEDED packets needed for path MTU discovery.
icmp_echo_ignore_all
If this value is non-zero, Linux will ignore all ICMP_ECHO
requests.
icmp_echo_ignore_broadcasts
If this value is non-zero, Linux will ignore all ICMP_ECHO
packets sent to broadcast addresses.
icmp_echoreply_rate
Maximum rate for sending ICMP_ECHOREPLY packets in response to
ICMP_ECHOREQUEST packets.
icmp_paramprob_rate
Maximum rate for sending ICMP_PARAMETERPROB packets. These
packets are sent when a packet arrives with an invalid IP
header.
icmp_timeexceed_rate
Maximum rate for sending ICMP_TIME_EXCEEDED packets. These
packets are sent to prevent loops when a packet has crossed too
many hops.

NOTES

As many other implementations don’t support IPPROTO_ICMP raw sockets,
this feature should not be relied on in portable programs.
ICMP_REDIRECT packets are not sent when Linux is not acting as a
router. They are also only accepted from the old gateway defined in
the routing table and the redirect routes are expired after some time.
The 64-bit timestamp returned by ICMP_TIMESTAMP is in milliseconds
since January 1, 1970.
Linux ICMP internally uses a raw socket to send ICMPs. This raw socket
may appear in netstat(8) output with a zero inode.

VERSIONS

Support for the ICMP_ADDRESS request was removed in 2.2.
Support for ICMP_SOURCE_QUENCH was removed in Linux 2.2.